U.S. Pat. No. 1,885,355, East German Pat. No. 56,796 (Chemical Abstracts 68:21742n) and Belousova et al. (Chemical Abstracts 65:16915n) teach the condensation reaction of 2-naphthol and p-phenylenediamine to produce N,N'-di-2-naphthyl-p-phenylenediamine: ##SPC1##
U.S. Pat. No. 1,885,355 teaches purification of the reaction product with methanol or ethanol, East German Pat. No. 56,796 teaches methanol, and both Belousova et al. and U.S.S.R. Pat. No. 140,065 (Chemical Abstracts 56:10066c) teach the use of ethylene glycol and water.
Excess 2-naphthol commonly used in the reaction of 2-naphthol and p-phenylenediamine may be found as an impurity in the crude reaction product, together with dark colored impurities and high trace amounts (up to 1000 ppm and more) of 2-aminonaphthalene. Excess 2-naphthol is desirably recovered and recycled to improve process efficiency. Dark colored impurities discolor polymer systems containing N,N'-di-2-naphthyl-p-phenylenediamine, and the discolored polymers may stain other polymers upon contact. 2-Aminonaphthalene is a well-known carcinogen and desirably is reduced to lowest possible levels by purification. Low molecular weight alkanols and ethylene glycol used by the prior art remove excess 2-naphthol satisfactorily but leave behind unsatisfactory amounts of dark colored impurities and generally also unsatisfactory amounts of 2-aminonaphthalene. A new purification method is desired which removes substantially all three impurities from a crude N,N'-di-2-naphthyl-p-phenylenediamine reaction product mixture.